The present invention relates generally to amplifier systems, and more particularly to a feedback arrangement for correcting distortion in an amplifier system.
Generally, audio amplifiers, or the like, are designed to amplify an input signal and apply it to a load with a minimum amount of distortion. In systems using inductive loads, a back EMF or voltage is generated by the load which is proportional to the inductance of the load times the rate of change of the current through the load. Heretofore, conventional amplifiers used to drive inductive loads have been unable to do so without introducing distortion into the output signal. This distortion is due to the time varying back EMF change which either boosts or reduces the output signal, thus introducing distortion.
One prior attempt to correct for distortion in inductive loads utilizes a fixed filter arrangement to provide a predistorted signal to the load. If the distortion characteristics of the load are known, the predistorted signal is shaped so as to interact with the load to produce an undistorted signal in a manner analogous to the concept of destructive interference.
However, this approach produces acceptable results only over a limited frequency range. The approach is unacceptable when used with devices such as shaker table drivers, magneto-strictive devices, and loudspeakers, or the like, which operate over wide frequency and harmonic ranges, and which are subject to varying loads over their operating ranges. Also, distortion in recording systems such as magnetic tape phonograph discs, or the like, may not be corrected by prior art systems.
For example, many prior art systems have attempted to correct for distortion in amplifier systems incorporating loudspeakers. Many factors affect the distortion-free performance of the loudspeaker including speaker cone mass, air resistance, and self-resonances. Nonlinearities introduced by these speaker properties result in changes in the back EMF of the speaker which boosts or reduces the amplifier output signal, producing unwanted distortion.
Prior attempts to correct for distortion include various types of sensors in the speaker to detect speaker motion. Typical of these sensors are light beam systems, extra voice coils, and tapped voice coils. The sensors generate error signals by comparing signals proportional to the speaker motion and the amplifier input signal, and applying the error signals to the amplifier in a feedback arrangement.
Typical of prior art systems utilizing feedback are U.S. Pat. No. 2,358,630, issued Sept. 19, 1944, for "Amplifier System"; U.S. Pat. No. 3,656,831, issued Apr. 18, 1972, for "Feedback Amplifier"; and French Pat. No. No. 7509775, issued Oct. 29, 1976 for "Loudspeaker Feedback Circuit."
The French patent is considered the most relevant prior art. This patent discloses a differential power amplifier driving a loudspeaker in series with a load resistor. Negative feedback is applied to the amplifier from a point between the loudspeaker and the load resistor. This patent also discloses using an RC filter arrangement as part of the feedback loop.
The arrangement disclosed in the French patent, however, may not provide distortion-free performance over the full speaker frequency range. Error signals produced by this arrangement are affected by possible resonances between the error signal-sensing circuit and the inductive load, since the circuit is connected directly to the load. Also, the strength of the error signal as fed back to the amplifier is generally not sufficient in strength to completely cancel the speaker distortion.
In addition, many of the prior art systems include many components, which adds to system complexity, cost and reduced reliability. None of the prior art systems mentioned hereinabove includes short circuit protection for the amplifier. Those systems incorporating sensors in the speakers introduce their own nonlinearities into the system, resulting in error signals which do not faithfully represent the speaker motion.